1. Field of the Invention
The present invention relates to a distortion compensating apparatus, and in particular to a distortion compensating apparatus for adaptively compensating a non-linear characteristic of a circuit having a non-linear distortion.
Recently, a linear modulation method tends to be used for enhancing utilization efficiency of frequency which is a resource in a wireless communication including a mobile terminal. In this linear modulation method, a non-linear characteristic of an input/output power characteristic in a circuit having a non-linear distortion such as a power amplifier, and a low efficiency of power utilization caused thereby are the issues. In order to solve these issues, it is important to compensate the input/output characteristic of the power amplifier to provide a distortion-free linear characteristic.
2. Description of the Related Art
Various methods including a feed-forward type method have been proposed as a method of realizing a power amplifier having a linear characteristic, one of which is an adaptive predistorter type compensating method having a feedback loop.
FIG. 19A shows an adaptive predistorter type distortion compensating apparatus 110 for compensating a non-linear characteristic of a power amplifier 300 by a feedback loop. This adaptive distortion compensating apparatus 110 is composed of an adaptive distortion compensating algorithm processor 130 and a multiplier 120.
FIG. 19B shows an input/output power characteristic of the power amplifier 300, where a linear region A1 indicating a proportional characteristic between an input power and an output power, and a non-linear region (portion indicated by a solid line) A2 in which the output power approaches its saturation are shown.
The adaptive distortion compensating algorithm processor 130 operates an adaptive algorithm in order to minimize an error between a reference signal 500 which is an input signal 500 and a feedback signal 710 which is an output signal 600 of the power amplifier 300, and calculates a distortion compensating coefficient 720. The multiplier 120 provides to an input terminal of the power amplifier 300 a signal 510 obtained by multiplying the input signal 500 by the distortion compensating coefficient 720.
Namely, the distortion compensating apparatus 110 preliminarily multiplies the input signal 500 by the distortion compensating coefficient 720 to be provided to the power amplifier 300, thereby compensating the non-linear distortion of the power amplifier 300. As a result, the utilization efficiency of the power amplifier 300 is enhanced.
FIG. 19C shows an effect of a distortion compensation, where an abscissa indicates a frequency and an ordinate indicates an amplitude (radiant power or voltage). A frequency characteristic of a radiant power before the distortion compensation is shown by a solid line. Radiant powers B1 and B2 are generated in bands C1 and C3 outside a band C2 of an input signal. The radiant powers (dashed lines) of the bands C1 and C3 after the distortion compensation are reduced to powers B3 and B4.
In order to prevent the effect by the distortion compensation in such a prior art distortion compensating apparatus from being lost, it is necessary to reduce an influence by the frequency characteristic held by an analog circuit except the non-linear distortion. By means of an equalization with a filter having an inverse characteristic of the frequency characteristic of the analog circuit including the power amplifier 300, the influence can be reduced to a certain degree.
Although an adaptive equalizer is required since the characteristic of the analog circuit substantially varies with temperatures and aging, an adaptive equalizer suitable for the adaptive predistorter type distortion compensating apparatus has not been developed so far.
It is accordingly an objective of the present invention to provide a distortion compensating apparatus for adaptively compensating a non-linear characteristic of a circuit having a non-linear distortion, whereby a frequency characteristic of the non-linear distortion circuit is adaptively equalized and also a computing amount required for this equalization is reduced.
In order to achieve the above-mentioned objective, a distortion compensating apparatus according to the present invention comprises: an adaptive distortion compensator for controlling an input signal to a circuit with a non-linear distortion by an adaptive algorithm so as to reduce an error between a reference signal and a feedback signal from the non-linear distortion circuit, thereby compensating the non-linear distortion; a digital filter connected between the adaptive distortion compensator and the non-linear distortion circuit, or to a pre-stage of the adaptive distortion compensator; a memory for preliminarily holding a filter coefficient group set for the digital filter; and an adaptive equalizing processor for adaptively selecting, from among the filter coefficient group set, a filter coefficient which makes an out-of-band power of the feedback signal reduced (to be set in the digital filter). (claim 1)
Namely, an adaptive distortion compensator controls an input signal of a non-linear distortion power amplifier by an adaptive algorithm so as to reduce an error between a reference signal and a feedback signal from a non-linear distortion circuit (analog circuit), e.g. a non-linear power amplifier (hereinafter, a non-linear power amplifier is occasionally described as an example of a non-linear distortion circuit; also, a non-linear power amplifier represents one including an analog circuit such as a peripheral filter and a mixer), thereby compensating the non-linear distortion of the non-linear power amplifier.
An adaptive distortion compensator which has been proposed fits for this adaptive distortion compensator, and e.g. an adaptive predistorter type distortion compensating apparatus can be used for it.
A digital filter is connected either between the adaptive distortion compensator and the power amplifier in cascade, or to a pre-stage of the adaptive distortion compensator.
An adaptive equalizing processor adaptively selects, from among a filter coefficient group held in a memory, a filter coefficient which makes an out-of-band (radiant) power of the feedback signal reduced (to be set in a filter).
Thus, it becomes possible to adaptively equalize the frequency characteristic of the power amplifier. Also, by using the filter coefficient preliminarily held in the memory, the computing amount can be reduced.
Also, in the present invention according to the above-mentioned invention, the adaptive equalizing processor may comprise a Fourier transform operation processor for performing a Fourier transform to the feedback signal to output an amplitude spectrum of the feedback signal, and an adaptive equalizing algorithm processor for selecting, from among the filter coefficient group, a filter coefficient which makes the out-of-band power of the feedback signal reduced by the adaptive algorithm based on the amplitude spectrum (to be provided to the digital filter). (claim 2)
Namely, a Fourier transform operation processor, e.g. a Fast Fourier Transform (hereinafter, occasionally abbreviated as FFT or FFT transform) operation processor performs a fast Fourier transform to the feedback signal to output an amplitude spectrum (hereinafter, occasionally referred to as amplitude characteristic) of the feedback signal.
An adaptive equalizing algorithm processor selects, from among the filter coefficient group, a filter coefficient which makes the out-of-band power of the feedback signal reduced by the adaptive algorithm based on the amplitude spectrum (to be provided to the digital filter).
Thus, it is possible to adaptively equalize the frequency characteristic of the analog circuit by the digital signal processing.
Also, in the present invention according to the above-mentioned invention, the adaptive equalizing processor may comprise a band-pass filter which passes a signal of a predetermined band outside a band of the feedback signal, a detector for detecting an envelope of the predetermined band-pass signal, an AD converter for performing an analog/digital conversion to the detected signal, and an adaptive equalizing algorithm processor for selecting, from among the filter coefficient group, a filter coefficient which makes the out-of-band power reduced by an adaptive algorithm based on a digitally converted signal (to be provided to the digital filter). (claim 3)
Namely, a band-pass filter passes a signal of a predetermined band outside a band of the feedback signal. A detector detects an envelope of this passing signal, and an AD converter converts this detected signal into a digital signal.
An adaptive equalizing algorithm processor selects, from among the filter coefficient group, a filter coefficient which makes the out-of-band power reduced by an adaptive algorithm based on a digital signal (to be provided to the digital filter).
Thus, it is possible to pass the out-of-band signal of the non-linear distortion circuit (through the band), and to detect the signal in an analog region.
It is to be noted that the detector and the AD converter can be a power detecting IC or the like which directly measures a power value.
Also, in the present invention according to the above-mentioned invention, the filter coefficient group may comprise a row of filter coefficients for setting an in-band amplitude characteristic of the input signal of the digital filter to a predetermined in-band gradient. (claim 4)
Namely, the filter coefficient group may comprise a row of filter coefficients for setting an in-band amplitude characteristic of the input signal of the digital filter respectively to e.g. linear gradients xe2x88x922 dB, xe2x88x921 dB, 0 dB, +1 dB, and +2 dB.
Also, in the present invention according to the above-mentioned invention, the out-of-band power may comprise an average out-of-band power of a plurality of measured instantaneous out-of-band powers. (claim 5)
Also, in the present invention according to the above-mentioned invention, the adaptive equalizing processor may measure a radiant power in one or more predetermined out-of-band measurement regions as the out-of-band power. (claim 6)
Namely, the adaptive equalizing processor may assume a measurement region of the out-of-band power to be e.g. a predetermined band region within a higher frequency band than the band of the input signal, a predetermined band region within a lower frequency band, a predetermined band region within both frequency bands, or the like.
Thus, the adaptive equalizing processor can set the measurement region of the out-of-band power according to the frequency characteristic of the non-linear distortion circuit.
Also, in the present invention according to the above-mentioned invention, the adaptive equalizing processor may adaptively select a filter coefficient which nulls a difference in out-of-band powers between two measurement regions measured using the same filter coefficient respectively. (claim 7)
Also, in the present invention according to the above-mentioned invention, the adaptive equalizing processor may select a maximum value within out-of-band powers measured for each filter coefficient respectively in a plurality of the measurement regions, and may adaptively select a filter coefficient which minimizes the maximum value. (claim 8)
Also, in the present invention according to the above-mentioned invention, the adaptive equalizing processor may obtain a simple average of out-of-band powers measured for each filter coefficient respectively in a plurality of the measurement regions, and may adaptively select a filter coefficient which minimizes the average. (claim 9)
Also, in the present invention according to the above-mentioned invention, the adaptive equalizing processor may obtain a moving average of out-of-band powers measured for each filter coefficient respectively in a plurality of the measurement regions, and may adaptively select a filter coefficient which minimizes the average. (claim 10)
Also, in the present invention according to the above-mentioned invention, the adaptive equalizing processor may obtain a weighted average of out-of-band powers measured for each filter coefficient respectively in a plurality of the measurement regions, and may adaptively select a filter coefficient which minimizes the average. (claim 11)
Also, in the present invention according to the above-mentioned invention, the adaptive equalizing processor may adaptively converge a filter coefficient of a reference in-band gradient to a filter coefficient which minimizes an out-of-band power, by repeating that when signs of differences in out-of-band power, measured by a reference in-band gradient or one or more compared in-band gradients in each measurement region, between adjoining in-band gradients are same, a compared in-band gradient having a larger absolute value of a difference between both out-of-band powers is assumed to be a subsequent reference in-band gradient, and when the signs are different from each other, a compared in-band gradient having a smaller absolute value of a difference between both out-of-band powers is assumed to be a subsequent reference in-band gradient. (claim 12)
It is to be noted that in order to execute the present invention, a compared in-band gradient (in-band gradient to be compared) has only to be selected which makes the out-of-band power at the time when the filter coefficient of the compared in-band gradient is set in the digital filter smaller than the out-of-band power at the time when the filter coefficient of the reference in-band gradient is set in the digital filter.
Also, in the present invention according to the above-mentioned invention, the adaptive equalizing processor may change a distance on a row of filter coefficients between a filter coefficient having a reference in-band gradient and a filter coefficient having a compared in-band gradient, corresponding to a feedback number counted after a start of an algorithm. (claim 13)
Also, in the present invention according to the above-mentioned invention, the adaptive equalizing processor may change distance on a row of filter coefficients between a filter coefficient having a reference in-band gradient and a filter coefficient having a compared in-band gradient, corresponding to an average out-of-band power value when the filter coefficient having a reference in-band gradient is applied. (claim 14)
Also, in the present invention according to the above-mentioned invention, the adaptive distortion compensator may comprise an adaptive predistorter type distortion compensating apparatus, and the non-linear distortion circuit may comprise a power amplifier. (claims 15 and 16)
It is to be noted that this non-linear power amplifier includes an analog circuit such as an amplifier peripheral filter and a mixer.